Madagascar corundums ( Sapphire & Ruby discovery )

AL2O3 Sapphire found in a metamorphized limestone. · Crystallographic properties of corundum: Trigonal.c = 1,3630 pp 0 86°4' ; pa1 = 122°26' Macle according to p (1011), often polysynthetic, accompanied by plans of separation, similar plans are also observed according to a1 (0001) unequal break to conchoïdal. Hardness 9. Density 3,95 to 4,1 Refractive index: 1.76 - 1.77 Uniaxial and optically négative.ng = 1,7675; Np = 1,7593 The faces a1 frequently show phenomenon of asterism, generally due to reflexions within separation p. Strong polychromatism , with following maximum ng. · Chemical properties: corundum is composed of pure alumina; its colouring is due to metallic oxide traces or inclusions. The color of sapphire, due to iron and titanium oxide, gives its best effect under daylight. Electric light makes it often dark. The color of ruby, due to chromium oxide, on contrary shows its most luminous red under electric lighting. The Corundum shows in Madagascar two different aspects: 1 stony and opaque crystals. 2 crystals of smaller size often transparent and usable as gem. In Madagascar, stony corundum comes from mica schists metamorphized by granite, as well as granitic veinules endomorphized and more or less deprived of quartz which injects those. Silimanite is a usual satellite of corundum in this type of deposit. One also finds some in eruptive rocks, syenites. Gems are generally found in alluvium, but come either from basaltic slags, or from metamorphized limestones or endomorpheous feldspatic rocks. 1 2 3 4 5 One distinguishes two principal types of corundum in Madagascar: Type I : isoceloedric, more or less acute accompanied or not by a small face a1 (0001) and more rarely by facets p (1011). Represented by figures 1 to 11, but which often become complicated in consequence of irregularity of development of some of their faces and by stacking with parallel axes of a great number of individuals. 6 7 8 9 10 11 12 13 Scalenoedron Stacking with parallel axes, Crystal supporting on one of its bases gutters at contours of face. a small rhombohedron p in parallel position. Type 2: Characterized by association of prism d 1 (1120) at a broad base, with which can associate isosceles ones, among which e 3 is most frequent, as well as rhombohedron p. The base of Malagasy corundum crystals very frequently shows scratches or triangular figures in relief, limited by p. Macle of blue corundum. Translucent violet - pink sapphire